`includes` method to check if a class includes a mixin

[ghost]

It would be quite helpful to add a mixins field to classes, adding mixins to this array when include is called, and an includes method on class instances that return true if the class of that object includes the specified mixin.

[ActivexDiamond]

I would love to see this! I implement on my fork and it comes in very useful. If anyone's interested, I would love to make a PR for it.